This application is based on and claims priority under 35 U.S.C. xc2xa7119 with respect to Japanese Application No. 2000-163095, filed on May 31, 2000, the entire content of which is incorporated herein by reference.
This invention generally relates to vehicle brake systems. More particularly, the present invention pertains to a vacuum servo unit used in an automobile brake system.
A known vacuum servo unit for an automobile is disclosed in a Japanese patent application published as Toku-Kai 2000-108880. The vacuum servo unit described in this application is accommodated with a solenoid actuator. An input member disposed in the vacuum servo unit is divided into a front portion and a rear portion. The front portion of the input member is adapted to contact a reaction member, while the rear portion of the input member is movable back and forth relative to the front portion. An elastic member is disposed between the front portion of the input member and a solenoid plunger (movable core), and is elastically deformable by relative movement of the front portion of the input member and the plunger. The aforementioned vacuum servo unit is self-operated by balancing the reaction force from the reaction member and the attraction force of the solenoid plunger. Accordingly, the vacuum servo unit is actuated to control a braking force continuously in response to the vehicle running condition even when the self-operation is performed.
In the above-described known vacuum servo unit, a continuous control of the entire output range of the vacuum servo unit is required. Therefore, the electric current consumption by the solenoid actuator may be increased, thus requiring an increase in the size of the solenoid actuator. In addition, the known vacuum servo unit needs to be relatively complex in structure to carry out fine output control such as inter-vehicle distance control to maintain a safe distance between automobiles.
In light of the foregoing, it would be desirable to provide a vacuum servo unit that is capable of carrying out carry an appropriate brake control corresponding to the situation.
A need exists for a vacuum servo unit which is capable of being self-operated for braking control without requiring the same amount of electric current consumption.
A need also exists for a vacuum servo unit which is capable of being self-operated for braking control while at the same time possessing a solenoid actuator of reduced size compared to the known unit.
According to an aspect of the present invention, a vacuum servo unit includes a housing, a movable wall, a power piston, an input member, an output member, a reaction member, a valve seat, an actuator and a force transmitting member. The movable wall is disposed in the housing for dividing the interior of the housing into at least to a front chamber and a rear chamber. The power piston is connected to the movable wall and is movable with the movable wall. The input member is disposed in the power piston and is movable in the forward and rearward directions by an input force relative to the power piston. The input member further has a front portion and a rear portion, with the rear portion of the input being movable in the forward and rearward directions with respect to the front portion. The output member is operatively connected to the power piston for outputting a forwarding force of the power piston in response to the movement of the movable wall. The reaction member is disposed between the input and the output member for transmitting a reaction force to the input member corresponding to an output force generated by the output member. The reaction member is engageable with an engaging portion of the front portion of the input member. The valve seat is movable in the forward and rearward directions in response to the movement of the input member. The actuator has a movable member that moves the front portion of the input member and the valve seat, and an actuating member. The actuating member serves for moving the movable member to control the distance between the engaging portion of the front portion and the valve seat in response to a driving force. The force transmitting member is disposed between the front portion of the input member and the movable member of the actuator and is deformable in response to relative movement of the front portion and the movable member. The vacuum servo unit can also be constructed to include a first axial space between the reaction member and the front portion and a second axial space formed between the front portion and the rear portion of the input member when the actuator is inactive.
In the vacuum servo unit of the present invention, xe2x80x9cjumping operationxe2x80x9d is performed until the condition arises in which a reaction force is transmitted to the input member corresponding to the output force generated by the output member. The output force generated by the xe2x80x9cjumping operationxe2x80x9d is called the output force by the jumping operation. When the actuator is inactive during a normal brake operation, the total amount of a clearance between the reaction member and the front portion of the input member and a clearance between the front portion of the input member and the rear portion of the input member is a clearance to perform the jumping operation. On the other hand, when the actuator is active, only the clearance between the reaction member and the front portion of the input member becomes the clearance to perform the jumping operation.
The force transmitting member can be disposed between the front portion of the input member and the movable member without an axial space between the front portion and the movable member. When the actuator is active, the force transmitting member is deformed by the rearward biasing force of the front portion of the input member and the biasing force of the movable member of the actuator after the reaction member is in contact with the front portion of the input member. The valve seat engaged with the movable member of the actuator is movable in the forward direction corresponding to the deformation resistance of the force transmitting member to introduce air into the rear chamber of the housing. Therefore, the valve seat is continuously movable corresponding to the forward movement of the movable member of the actuator.
With the present invention, the vacuum servo unit is designed so that the output force of the jumping operation by self operation of the actuator is smaller than the output force of the jumping operation by the normal braking operation by the driver. Therefore, a fine control of the output force and a sensitive braking operation such as an inter-vehicle distance control can be carried out by the vacuum servo unit of the present invention.
According to another aspect of the present invention, the front portion of the input member is retracted in the rearward direction relative to the power piston by the reaction member when the actuator is active. In this case, it is favorable that the retracted distance of the front portion relative to the power piston is limited to a predetermined value.
When the front portion of the input member is retracted by the reaction member, the retracted distance of the front portion relative to the power piston is controlled to be less than the predetermined value. When the driving force by the actuator reaches the predetermined value which is the maximum driving force or less than that, the retraction of the front portion of the front member relative to the power piston is stopped, wherein the driving force supplied to the force transmitting member is not increased any more. Therefore, the vacuum servo unit of the present invention can generate the maximum output force when the driving force generated by the actuator is the predetermined value or larger.
The self operation of the actuator according to the vacuum servo unit of the present invention is continuously performed within a small output force corresponding to the driving force generated by the movable member of the actuator. Further, compared with self operation to continuously control the entire output range of the vacuum servo unit, the electric current consumption by the vacuum servo unit of the present invention is decreased. Further, the maximum output force can be generated when the driving force by the actuator is the predetermined value or larger. Thus, the vacuum servo unit of the present invention can be appropriately utilized for an emergency braking assist which requires the maximum output force.
According to a further aspect of the present invention, it is preferable that an axial space be provided between the front portion of the input member and the force transmitting member or between the force transmitting member and the movable member. The output force of the jumping operation with the actuator being active thus becomes larger than that with the normal braking operation by the driver.